Drilling fluids, often referred to as drilling muds, play an important role in the drilling of subterranean wells. Such fluids include drilling muds, completion and work over fluids including non-dispersed drilling muds, dispersed drilling muds, calcium treated drilling muds, drilling muds incorporating polymers, drilling muds prepared from fresh or brine, oil-based drilling muds and synthetic drilling muds. Other fluids used in the drilling of oil and gas wells include bactericides, calcium removers, corrosion inhibitors, defoamers, emulsifiers, filtrate reducers, flocculants, foaming agents, lost circulation materials, lubricants, pipe-freeing agents, shale control inhibitors, and surface active agents. A discussion of the various types of drilling, completion and workover fluids used in the oil and gas well drilling industry can be found in the June 1994 issue of “World Oil”, which is incorporated herein by reference.
Such fluids are formulated to have specific properties for the required functions and characteristics for their intended purpose. For example, a drilling fluid should circulate throughout the well and carry cuttings from beneath the bit, transport the cuttings up the annulus of the borehole, and allow their separation at the surface. At the same time, the drilling fluid is expected to cool and clean the drill bit, reduce friction between the drill string and the sides of the hole, and maintain stability in the borehole's uncased sections. The drilling fluid should also form a thin, low permeability filter cake that seals openings in formations permeated by the bit and act to reduce the unwanted influx of formation fluids from permeable rock.
Drilling fluids are typically classified according to their base material. In oil based fluids, solid particles are suspended in oil, and water or brine may be emulsified within the oil. The oil is typically the continuous phase. In water based fluids, solid particles are suspended in water or brine, and oil may be emulsified in the water. Water is typically the continuous phase. Pneumatic fluids are a third class of drilling fluids in which a high velocity stream of air or natural gas removes drill cuttings.
Drilling fluids are typically transported and stored in such things as boat tanks, bilges and holds; barges; mobile and land based tanks; cutting boxes or containers; and related vessels (sometimes collectively referred to herein as “vessels”). Such vessels are used to carry different types of drilling fluids and each vessel must be cleaned of the last previous drilling fluid before the next drilling fluid can be introduced. While various procedures are practiced for cleaning such vessels of drilling fluid residue before introducing another, there remains a need in the art for improved and more efficient cleaning operations.